ALSA: core: memalloc: add page alignment for iram

Since mmap for userspace is based on page alignment, add page alignment
for iram alloc from pool, otherwise, some good data located in the same
page of dmab->area maybe touched wrongly by userspace like pulseaudio.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1608221747-3474-1-git-send-email-yibin.gong@nxp.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>